The use of industrial waste materials as additives in the manufacture of ceramic products has been attracting a growing interest from researchers in recent years and is becoming common practice. This work describes the changes in the behavior of the clay material used in a red-clay industry due to additions of a granite and marble sludge, produced in an ornamental stone processing industry in Rio Grande do Norte-Brazil. Mixtures of clay and waste material (10–50 wt.%) were uniaxially pressed and sintered at temperatures ranging from 950 to 1150 °C. Results from chemical and mineralogical analysis (XRD and XRF), thermal analysis (DTA, TG and dilatometry), apparent density, water absorption and flexural strength, show that the granite and marble sludge can be added to the clay material with no detrimental effect on the properties of the sintered red-clay products. The reject acts as a flux agent, reducing the sintering temperature of the clay material.